When modding both cars under the same rule (FIA GT1-rule)
996-GT2-Evo versions (by PSI Motorsports and others) were faster than C5's but couldn't compete with C5 or C6 racing recreations after the FIA turbo air restrictor rule changes (Porsche doesn't have now car qualified by any race car tuning company to GT1 cathegory).
Therefore this also doesn't tell which is better (FIA regulations for turbo restrictors are made for killing Porsche not to win every series, unfortunately), but it gives an idea of strengths of a big naturally aspirated engine (max 8,0L N/A allowed) against biturbo (max 4,0L charged allowed) against the turbo engines...
What comes to driveability, at least race ready FIA-C6 is an extremely great car to drive fast on corners. Of course, if I can choose, I would take GT2 at any version, but track times are track times, C6 is under the existing rules faster than any GT-Porsche... .

Interesting. I was wondering why the vettes are classified so differently than Porsches in racing. It pisses me off frankly that the Vettes and Porsche are always in different classes and different power levels. I mean this is the greatest rivarly between two manufacturers, IMO. More than Porsche and Ferrari, because those two companies both make true exclusive European sport cars, so they have something in common at least.

The battle between the Americans and Germans is just fun to me. And when the Porsche goes faster, I like it that much better.